A titanium or titanium alloy has recently attracted much attention as a metallic member hardly causing metallic allergy and friendly to human bodies, and consequently, has been utilized for such decorative articles as represented by a wristwatch, eyeglasses, accessories, and the like.
It has been pointed out, however, that the titanium or titanium alloy has a problem of susceptibility to scratches due to its low surface hardness, and a tendency of degradation in the quality of appearance after use for long duration.
Many attempts have so far been made to apply various surface hardening treatments to the titanium or titanium alloy in order to overcome such problems.
Conventional methods of applying a surface hardening treatment to the titanium or titanium alloy can be broken down into a method of coating the surface of a metal with a hard film, and a method of hardening the metal itself.
Among methods of coating the surface of the metal with the hard film, there are known a wet process as represented by electroplating, and a dry process as represented by vacuum deposition, ion plating, sputtering, plasma CVD, and the like.
Any of these methods, however, has had a problem with adhesion between the titanium or titanium alloy and the hard film, and a drawback of the hard film being prone to peeling off.
On the other hand, ion implantation, ion nitriding, gas nitriding, carburizing, and the like are well known among methods of applying a hardening treatment to the titanium or titanium alloy itself. There is no risk of a hard layer, formed at the surface of the titanium or titanium alloy by means of such surface hardening methods, peeling, as in the case of the hard film described above.
However, conventional surface hardening methods have had shortcomings of low productivity due to long processing time, and also a drawback of degradation in the quality of appearance because the surface of the titanium or titanium alloy becomes coarse due to growth of crystal grains taking place in the surface owing to high processing temperatures. It has further been pointed out that since it is difficult to form a hard layer in regions deep down from the surface, there is a tendency for the quality of appearance to deteriorate due to scratches and the like incurred during use, presenting a major problem.
The invention has been developed in light of the circumstances described above. That is, it is an object of the invention to provide a titanium or titanium alloy superior in the quality of appearance, and having hardness sufficient to withstand large impact.
Another object of the invention is to provide a surface treatment method whereby a titanium or titanium alloy is provided with such properties as described in the foregoing.